a. Field of the Invention
This invention pertains to piezoelectric resonators and methods for the fabrication of piezoelectric resonators. More particularly, this invention pertains to thin film piezoelectric resonators and methods for fabricating thin film resonators.
b. Description of the Prior Art
Piezoelectric resonators typically are used for the control of the frequency of oscillation of oscillators or as filters or components of filters that are used to select those frequency components of an electrical signal that lie within a desired band of frequencies while eliminating or attenuating those frequency components that lie outside the desired band of frequencies or that lie within specific undesired bands of frequencies.
At ultra high (UHF) and microwave frequencies, piezoelectric resonators have been fabricated using thin-film techniques such as those described in "Development of Miniature Filters for Wireless Applications", Lakin, Kline, McCarron, IEEE Trans. Microwave Theory and Techniques, Vol. 43, No. 12, December 1995, pp. 2933-2929; "Thin Film Bulk Acoustic Wave Filters for GPS", K. M. Lakin, G. R. Kline, and K. T. McCarron, 1992, Ultrasonics Symposium Proc. pp. 471-476; High-Q Microwave Acoustic Resonators and Filters," by Lakin, Kline and McCarron, IEEE Trans. on Microwave Theory and Techniques, Vol. 41, No. 12, December 1993, p. 2139. One such method of fabricating piezoelectric resonators consists of first depositing a layer of conducting material upon the upper surface of a non-conducting substrate and then removing portions of the conductor by etching so as to leave a desired conducting pattern which forms a lower electrode. The upper surface of the conductor is then used as a substrate upon which is deposited a layer of piezoelectric material. The upper surface of the piezoelectric material is then used as a substrate upon which is deposited another layer of conducting material. Portions of the upper-most conducting material are then removed by etching so as to leave a second conducting pattern of conductor which forms an electrode on the upper surface of the piezoelectric material. Each portion of the layer of piezoelectric matter that is sandwiched between the two electrodes, together with these bounding conductors forms the piezoelectric resonator. In some prior art devices, these resonators are supported upon one or more layers of material that provide, in effect, either a fixed surface having a high mechanical impedance to vibration, or a "free" surface having a low mechanical impedance to vibration. See, e.g. U.S. Pat. Nos. 3,414,832 and 5,373,268. In some prior art devices, areas of the substrate located beneath the resonators are removed so as to leave the resonators as thin membranes. See e.g., U.S. Pat. No. 4,456,850, which patent also discloses the use of multiple layers of piezoelectric material having offsetting temperature coefficients, that are combined to provide a resonant frequency for the resonator that is relatively insensitive to temperature variations. FIG. 1 depicts such a resonator of the prior art that includes electrodes 11 and 13 located respectively below and above a layer of piezoelectric material 12. Electrodes 11 and 13 typically are made of gold or aluminum.
A major factor in determining the performance of such resonators is the magnitude of the electromechanical coupling coefficient, K.sup.2 (i.e. "K" squared), which coupling coefficient relates the strength of the electric field that is generated within the piezoelectric material, when it is mechanically deformed, to the amount of mechanical deformation. A resonator that utilizes a piezoelectric material that has a higher value of K.sup.2 can, other things being equal, exhibit lower loss and higher quality or "Q". Such higher Q resonators typically provide better frequency control than lower Q resonators. Such high Q resonators also can be used as part of filters that have wider bandwidths and lower losses than filters using resonators that have a lower value of Q. Resonator K.sup.2 and Q are effective values derived from the electrical impedance of the resonator through measurement and modeling. See, e.g. "High-Q Microwave Acoustic Resonators and Filters", Lakin, Kline and McCarron, IEEE Transactions on Microwave Theory and Techniques, Vol. 41, No. 12, December 1993, p. 2139. Accordingly effective K.sup.2 and Q are determined by the composite of materials that make up the resonator.
Of the materials that are used to fabricate thin-film resonators, the piezoelectric ceramics such as barium titanate, lead zirconate titanate, lithium niobate, zinc oxide, lithium tetra borate and aluminum nitride have relatively high values for the electromechanical coupling coefficient K.sup.2. However, in order to take advantage of the high value of K.sup.2, a substantial portion of the crystalline structure within the piezoelectric material must be oriented in one, desired direction so that the piezoelectric layer of crystals can, in bulk, also exhibit the same high value of K.sup.2. In order to obtain a piezoelectric layer of zinc oxide or aluminum nitride that has a substantial degree of uniformity in the orientations of its crystals, the piezoelectric layer usually is deposited upon a layer of material, e.g. gold or aluminum, which, itself, also has a substantial degree of uniformity in the orientations of its crystalline structure.
A substantial degree of uniformity of crystal orientations within the gold or aluminum electrode 11 can be obtained by depositing the gold or aluminum layer upon a substrate in circumstances in which the gold or aluminum atoms have a high mobility during the deposition process. Such high mobility for the gold or aluminum atoms can be obtained if, during the deposition process, the substrate upon which the gold or aluminum is deposited is held at an elevated temperature that is less than the melting temperature for the gold or aluminum.
Gold typically has been used as the substrate for a zinc oxide piezoelectric layer and aluminum typically has been used as the substrate for an aluminum nitride piezoelectric layer. Because zinc oxide has a higher value of K.sup.2 than aluminum nitride, a piezoelectric layer of zinc oxide, in which the orientations of the crystalline structure have a substantial degree of uniformity, should exhibit a higher value for K.sup.2 and equivalent Q of a similar layer of aluminum nitride. However, in prior art devices, the gold, that was used as the electrodes and as the substrate for the zinc oxide, introduced excessive mechanical losses at UHF and microwave frequencies and which resulted in lower Q resonators and poorer overall performance as compared to devices that utilized a piezoelectric layer of aluminum nitride and aluminum electrodes.
Attempts to fabricate useful resonators by depositing a piezoelectric layer of zinc oxide upon an aluminum substrate were not successful because the process for depositing the zinc oxide utilized an oxygen rich environment, which oxydized the surface layer of the aluminum substrate during the initial film nucleation stage of the deposition process. Although the underlying layer of aluminum had a highly oriented crystalline structure, the oxydized surface layer of aluminum acted as a screen between the underlying layer of aluminum and the zinc oxide that was being deposited upon the oxydized surface layer of aluminum. In addition, zinc oxide was typically sputter deposited at substrate temperatures near 300 degrees C., in which circumstance the unprotected aluminum films undergo a structural change known as hillocking which roughens the surface and reduces the effectiveness of the aluminum as a nucleation layer. As a consequence the uniformity of the orientations of the crystals within the zinc oxide was relatively low and the quality of the resonators was poor.
Attempts to fabricate useful resonators using zinc oxide were further complicated by the fact that the chemicals, such as mild acids and bases, that are ordinarily used in the fabrication of integrated circuits, attack zinc oxide.